Registration of lightning strike in a wind turbine

ABSTRACT

The invention relates to aspects of a method of registering lightning strikes in the blade of a wind turbine, use of such method, a system of exercising the method. The method according to the invention of registering lightning strikes involve that the method comprises that the lightning strike is captured in a receptor arranged in the blade ( 5 ) of the wind turbine, and the lightning current is conducted completely or partially through at least one electric resistor ( 10 ), thereby heating it, and that it is registered at least that the temperature of the resistor is or was at some point increased. The heating of the electric resistor, ie the increase in temperature, provides simple and reliable registration. This can be extended to also comprise measurement of the amount of energy contained in the lightning current in a very robust and hence very suitable manner for the task. By the invention it is acknowledged that if a lightning current is conducted through a suitable electric resistor, this will “capture” a share of the amount of energy in the same manner as eg an electric radiator or a furnace, ie the resistor is heated by the current as a function of the voltage and the duration and hence of the amount of energy.

BACKGROUND OF THE INVENTION

The invention relates to aspects regarding a method of registeringlightning strikes in the blade of a wind turbine, a system of exercisingthe method, exercise on a wind energy plant and a suitable resistorelement in the same context.

It is commonly known to equip high structures, such a high-risebuildings, chimneys, antennae and wind energy plants with lightninggrounding devices. By means of such lightning grounding devices it isensured that lightning current is grounded in a responsible manner andwithout damage to the structure.

In case of eg the blades of a wind turbine, a strike of lightning can becaptured by means of receptors, as described more specifically in eg WO0079128 and WO 0248546, incorporated herein for reference. Essentially,receptors are metallic elements arranged strategically on the blade andbeing in connection with one or more lightning grounding cables, throughwhich the lightning currents are grounded.

In order to enable estimation of the force of a lightning strike in theblade of a wind turbine, a so-called lightning registration card of asize similar to that of a credit card can be mounted in the blade, asdisclosed in EP 0 663 598, wherein a magnetic field generated by alightning leaves traces on the magnetic strip of the lightningregistration card. In this manner it is possible to read the maximallightning current to which the blade was exposed.

In order to enable this reading, however, it is necessary to stop thewind energy plant and dismount the lightning registration cards inquestion that may be mounted eg within each blade of the wind turbine.The lightning registration cards can subsequently be read by means ofspecial equipment, whereby the maximal value of the lightning currentcan be determined. However, it is not possible to determine neither theduration of the lightning strike nor when it happened.

Precisely the duration and point in time of the lightning strike aredesirable items of information—on the one hand to enable assessment ofthe risk of damage, if any, to the blade and, on the other, inconnection with insurance issues concerning blades of wind turbines thathave been damaged by lightning strikes.

If a lightning current of a certain magnitude—eg 50 kA—is dischargedduring a few microseconds, ie the duration of the lightning current isquite brief, it is likely that no damage is done, as there has not beentime enough for any heating worth mentioning of the blade and the aircontained in the blade to have occurred. This is assumed to be linkedwith the fact that, obviously, when the duration is short, the energyamount is limited. Conversely, in case of a less powerful lightningcurrent of eg 30 kA discharged during a full second, a more powerfulheating takes place with ensuing possible damage to the blade. Thereason for this is assumed to be the discharge time which results in theamount of energy being many times greater than the more powerful strikesof lightning of shorter duration mentioned earlier.

It is known to measure lightning current and discharge time by means ofan oscilloscope or digital recording equipment; however, this equipmentis expensive laboratory equipment which is moreover not suitable formounting in eg the blade of a wind turbine.

BRIEF SUMMARY OF THE INVENTION

It is a starting point of the invention to provide a safe and robustmethod of registering the presence of a lightning current in the bladeof a wind turbine, including that it has occurred and when it occurred.It is a second starting point to provide a method for quantification ofthe energy amount in a lightning strike in the blade of a wind turbine.It is yet a starting point to provide a wind energy plant, wherein theabove-mentioned methods can be exercised, and wherein strikes oflightning can be registered and the amount of energy measured. It is afurther starting point to provide systems for determining the magnitudeof lightning strikes and for registering lightning strikes, wherein thesystems can be built temporarily or permanently into a wind energyplant, including into the blade of a wind turbine. Yet a starting pointis to provide a resistor element which is suitable for use whenexercising the above-mentioned methods and robust and comparativelyinexpensive and can be built integrally with a wind energy plant,including the blade of a wind turbine.

Novel aspects of a method according to the invention for registering atleast one lightning strike in the blade of a wind turbine involve thatthe methods comprise that the lightning strike is captured by a receptorin the blade of the wind turbine, from where the lightning current iscompletely or partially conducted through at least one electricresistor, thereby heating it, and that the lightning strike isregistered on the background of the increase in temperature of theresistor. It is hereby possible to at least verify, immediately orlater, that a lightning strike has occurred, on the basis of theregistration.

According to a preferred embodiment the registration may comprise thatat least one characteristic of at least one lightning strike isdetermined on the basis of the increase in temperature of the resistor,which characteristic comprises the energy amount contained in thelightning strike determined on the basis of a measurement of themagnitude of the increase in temperature.

The heating of the electric resistor, ie the increase in temperature,yields a simple and reliable measurement of the amount of energy whichis simultaneously very robust and therefore very suitable for the task.Lightning currents are usually brief and are of a very fluctuatingnature, and therefore it is difficult and presupposes expensive andcomplex equipment to follow a lightning current throughout its durationand succeed in capturing all fluctuations in order to thereby the ableto calculate the discharged amount of energy by integration. By theinvention it is acknowledged that if a lightning current is conductedthrough a suitable electric resistor, this will “capture” a portion ofthe energy amount in the same manner as eg an electric radiator orfurnace, ie the resistor is heated by the current as a function of thecurrent intensity and the duration and hence as a function of the energyamount, and therefore the increase in temperature directly expresses theamount of energy in the lightning current in question. Hereby it isavoided to also have to conduct the lightning current directly throughthe measurement equipment; thus, it suffices to perform measurements onthe resistor, which can be done with comparatively inexpensive andsimple equipment, eg an electronic thermometer. As a minimum the methodcan be used for ascertaining whether there is a difference in the amountof energy in two lightning currents.

According to yet a preferred embodiment, the registration may comprisethe point in time of the lightning strike determined on the basis of ameasurement of the point in time of the increase in temperature. Therebyassociated values can be used eg for comparison to values measured by egpublic institutions that monitor and register lightning strikes.Moreover the information regarding the strike of lightning and the pointin time it struck will be relevant in insurance matters.

According to a further preferred embodiment the determination of theenergy amount contained in the lightning strike is determined on thebasis of the magnitude of the increase in temperature of the resistor byuse of a pre-defined ratio coefficient defining the ratio of increase intemperature in the electric resistor to the amount of energy of thecurrent conducted through the electric resistor. Hereby it is enabled,on the basis of the increase in temperature, to find the actual amountof energy and not merely to see whether a lightning current contains asmaller or larger amount of energy than some other lightning current—andto do so without complex laboratory measurement equipment, such as anoscilloscope. It is possible to find eg a ratio coefficient by using anumber of simulated strikes of lightning having known, varying energyamounts. Such simulated strikes of lightning are conducted through theelectric resistor, following which the ensuing increases in temperaturein the resistor are measured. Then eg an average value of the ratio ofthe increases in temperature in the resistor to the actual amounts ofenergy is used to determine the ratio coefficient.

A preferred use of the method may comprise that the method is exercisedon a wind energy plant, wherein at least the measurement of the increasein temperature is used as indicator for assessing the potential extentof damage of a lightning strike in the blade of a wind turbine. Herebyit is accomplished that, on the basis of knowledge of previous strikesof lightning and damage on blades resulting there from, can use theincrease in temperature of the resistor to provide an indication ofwhether a lightning strike has occurred in the blade, and, in theaffirmative, which damage it has done. The indication may also give riseto immediate maintenance, service and/or repair being implemented or toindicate whether it can wait. This enables savings on costs ofmaintenance and service, which—in particular in case of off-shore windenergy plants—are of a considerable magnitude; but being, of course,also of relevance in case of other locations.

Novel aspects of a wind energy plant comprising means for grounding alightning current, including at least one receptor and at least onegrounding connection from the receptor to an external connection toground involves that the wind energy plant comprises means for measuringan increase in temperature in at least one electric resistor, where theresistor is connected to the receptor or to the grounding connection ina position between the receptor and the grounding, preferably by beinginserted serially in the grounding connection and inserted seriallybetween the grounding connection and the receptor, respectively, or bybeing incorporated in a measurement shunt, measuring bridge or otherparallel circuit connected to the grounding connection or to thereceptor. Hereby the electric resistor is integrated in the plant andthe entire lightning current, or a pre-defined share thereof, will beconducted there through, when or if a lightning strike occurs in theplant and it is captured by the at least one receptor. Herebymeasurement of the increase in temperature can be performed as close aspossible to the site of strike, whereby it is very likely that theentire lightning current is there and is hence not reduced by transfersto other eg conductive objects, meaning that the measurement isaccurate. The increase in temperature is determined by the amount ofenergy contained in the lightning current, cf above, and is an indicatorboth that a lightning strike has occurred and whether there is a risk ofthe plant or parts of the plant having suffered any damage. Receptorsand grounding connections are often used in wind energy plants forprotection against lightning strikes, and the introduction of theresistor and means for measuring an increase in temperature of anelectric resistor and means for monitoring and registering the increasein temperature are this comparatively simple adaptations that arereadily performed for implementation on the plant. The implementationmay occur both on existing plants and on new plants.

According to one embodiment the resistor and the receptor can becombined or made in one piece whereby there is the shortest distancepossible between the site of strike and the site where the increase intemperature is registered and measured, respectively. Hereby the risk isreduced of a part of the lightning current being deflected by transferbefore it reaches the resistor.

According to a preferred embodiment the at least one electric resistorcan preferably be arranged in at least one blade of a wind turbine ofthe wind energy plant. Receptors for capturing lightning strikes areoften made integrally with the blades, where the receptor is in aposition close to the site of strike.

According to a further preferred embodiment the wind energy plant maycomprise a number of receptors, which receptors are each connected to anexternal connection to ground; and wherein at least one electricresistor is connected to each receptor or to the grounding connectionbetween each receptor and the connection to ground to which the receptoris connected; and wherein the wind energy plant comprises means formeasuring an increase in temperature in each electric resistor. By thisconfiguration it is possible to read whether a lightning has struck theblade in one receptor or whether the lightning has struck severalreceptors. However, it is not very likely that a lightning will haveseveral foot points/strike sites during discharge and hence hit severalreceptors. Alternatively the arrangement can be used for determiningwhich receptor was struck by lightning.

In specific embodiments the wind energy plant may comprise means fordetermining the amount of energy in the lightning strike on the basis ofthe size of the increase in temperature and for determining the point intime when the lightning struck on the basis of the point in time of theincrease in temperature. Moreover, the wind energy plant may comprisemeans for storing at least one of the parameters comprising the measuredincrease in temperature, the determined amount of energy and thedetermined point in time.

These data can be used to substantiate where, when and how comprehensivewere the respective energy discharges resulting from lightning strikes.Thus it is possible to ia accumulate statistic material of when a bladeis to be checked for damage due to lightning. For instance, it isperceivable that the part of the blade that is most proximate to the hubof the wind turbine is able to tolerate more or larger strikes oflightning than eg the tip of the blade before servicing is required. Bya protocol, ie stored data, of lightning strikes in the relevant bladeit is thus possible to plan servicing more accurately, and likewisedocumentation vis-à-vis an insurance company can also be provided veryaccurately.

According to a further preferred embodiment means for measuring theincrease in temperature in the electric resistor may comprise anelectronic thermometer comprising a thermo-element, which thermo-elementis arranged in heat-conductive contact with the electric resistor. Anelectronic thermometer is an inexpensive and robust component that takesup comparatively little space and can hence be integrated anywhere on awind energy plant, including in a blade.

According to yet an embodiment, means for measuring the increase intemperature in the electric resistor may comprise an infraredthermo-sensor and a cameral for infrared recording, respectively, meansfor measuring a temperature-related change in resistance in the electricresistor, a non-touch temperature sensor, an optical fibre or other kindof equipment for measuring an increase in temperature. Any of said kindsof equipment for measuring an increase in temperature or change intemperature are suitable for implementing the invention.

According to a preferred embodiment at least the electric resistor canbe essentially enshrouded in thermally insulating material. Hereby it isensured that the increase in temperature is not influenced by thesurroundings, whereby the measurement becomes more accurate.

According to yet a preferred embodiment the wind energy plant maycomprise means for monitoring and storing registrations of lightningstrikes, including optionally characteristics of lightning strikes,which means may comprise a computer unit arranged in direct or wirelessconnection with means for measuring the increase in temperature, whichcomputer unit is preferably arranged in or at the wind energy plant,including in the blade of a wind turbine. Use of a computer unitprovides several advantages, including that processing of measured data,and likewise storage of data, runs smoothly, and likewise it is enabledto externally, eg via the Internet, collect data from the computer unit,eg for use in the planning of servicing.

According to a further preferred embodiment the wind energy plant maycomprise means for alerting or halting the wind energy plant in theevent of a given increase in temperature in the resistor. This can beused to increase plant safety.

According to yet a preferred embodiment, the wind energy plant maycomprise means for dispatching an electronic message, which message maycomprise data relating to the increase in temperature. Hereby eg anexternal recipient can be advised that a lightning strike has occurredand where, when and how powerful it was.

According to yet a preferred embodiment the wind energy plant maycomprise means for registering a lightning current, including alightning registration card comprising at least one magnetic strip.Hereby it is possible to also register the magnitude of the maximallightning current and not merely the amount of energy.

Novel aspects of a system for use in the registration of at least onestrike of lightning in the blade of a wind turbine, said systemcomprising means for grounding a lightning current, including at leastone receptor for mounting in the blade of the wind turbine and at leastone lightning grounding cable coupled to the receptor, involve that thesystem comprises means (16) for measuring an increase in temperature inat least one electric resistor (10), wherein the at least one resistoris adapted for being connected to the lightning grounding cable or tothe receptor in such a manner that the resistor will be heated by alightning current. Hereby the system is suitable for determining theproperties of a lightning strike and is able to give in-situquantification of the amount of energy present in the lightning current,see above regarding the correlation between amount of energy containedin lightning current and the ensuing increase in temperature in theresistor. Thus, in addition to the safety aspect with regard to harmfuleffects of lightning strikes, the system is able to register that alightning strike has occurred.

According to a preferred embodiment the resistor can be insertedserially in the grounding connection and inserted serially between thegrounding connection and the receptor, respectively, or be incorporatedin a measurement shunt, measuring bridge or other parallel circuitconnected to the grounding connection or a receptor. Hereby it is anoption to let the entire lightning current or merely a pre-defined sharethereof be conducted through the resistor.

Novel aspects of a resistor element adapted for conducting a lightningcurrent and heating due to such current involve that the resistorelement is configured essentially as an elongate object having at itsend an increased expanse transversally to its longitudinal axis. Therebythe resistor element can be provided at its ends with internal threadedholes for eg connector elements for lightning grounding cable without anensuing undesirable reduction in the conductivity or increase in theelectric resistance of the resistor element. Simultaneously a limitedexpanse is provided transversally to the longitudinal direction, wherebythe element is readily integrated since such configuration is not veryspace-consuming.

According to a preferred embodiment the resistor element can beessentially rotationally symmetrical about its longitudinal axis andcomprise an opening adapted for receiving a thermo-element. Thus, theresistor element is suitable for manufacture ia by turning wherebyprecise measurements are readily accomplished, which is convenient forthe manufacture of a number of identical elements, including eg standardelements with very uniform electric resistance. By arrangement of thethermo-element in an opening in the resistor element it is possible toaccomplish good thermally conductive contact, which is necessary for aprecise measurement. The thermally conductive contact can moreover beenhanced by use of heat-conductive paste.

According to yet a preferred embodiment the resistor element can bemanufactured essentially from steel, preferably stainless steel.Stainless steel has a thermal conductivity which is comparatively lowcompared to carbon steel. The resistor element can also be manufacturedfrom other electrically conductive materials, such as eg tungsten orcarbon. Stainless steel is a convenient choice due to its poor heatconductivity and its large thermal capacity which makes it easy towithhold the deposited amount of energy in the resistor element, wherebya very accurate temperature reading is enabled.

In the following the invention will be described in further detail bymeans of figures that give examples of exemplary embodiments of theinvention, wherein:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 schematically shows a wind energy plant with three bladesequipped with receptors;

FIG. 2 shows an embodiment of the blade of a wind turbine according toan aspect of the invention;

FIG. 3 shows an alternative embodiment of the blade of a wind turbineaccording to a further aspect of the invention; and

FIG. 4 shows a resistor element having a lightning grounding cable and athermo-element mounted thereon.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a wind energy plant 1 of the type known within the trade asa “wind turbine according to the Danish Concept”. Such turbine 1typically comprises a tower 2, a nacelle 3 with gear and generator (notshown) and a rotor consisting of a hub 4 and three wind turbine blades5. On the blades 5 and at their outer side receptors 6 are provided thatare, via one or more lightning grounding cables, electrically connectedto ground. The receptors 6 are metal elements that are mounted at thesurface of the blade 5, to the effect that the lightning can be capturedon or formed opposite these specific positions. On the shown windturbine 1, five receptors 6 are mounted on each blade 5, but of coursethe number can be varied from one blade type to another, depending eg onsize. Also the tower 2 and/or the nacelle 3 can be equipped with meansfor capturing lightning.

FIG. 2 shows a blade 5 having five receptors 6 distributed across thelongitudinal direction of the blade. These receptors 6 are connected tothe same main lightning grounding element 7, typically constituted by acupper cable or the like, that is grounded 9 via a system 8 fordetermining the magnitude of the amount of energy contained in one ormore lightnings.

By the same configuration of receptors 6, lightning grounding cable 7and connection to ground 9, means for protecting against lightning, thesystem 8 can be exchanged for an alternative system that comprises meansfor registering an increase in temperature in at least one electricresistor that is, via connection to lightning grounding cable 7,inserted between at least one receptor 6 and the connection to ground 9,whereby the resistor will be heated by a lightning current deriving froma lightning strike into a receptor 6. An increase in temperature inexcess of the usual variation during the day deriving from heating bythe sun will thus be an indication that a strike of lightning hasoccurred.

FIG. 2 shows an embodiment of the mentioned system 8 for determining themagnitude of the amount of energy contained in a lightning strike. Thesystem 8 comprises a resistor element 10 used as an electric resistorand being connected to the lightning grounding cable 7 and theconnection to ground 9. In the various embodiments, the resistor element10 can be connected to the receptor 6 or to the grounding connection 7in a position between the receptor 6 and the connection to ground 9,preferably by being inserted serially into the grounding connection 7and serially inserted between the grounding connection 7 and thereceptor 6, respectively, or by being incorporated into a not shownmeasurement shunt, measuring bridge or other parallel circuit connectedto the grounding connection 7 or to the receptor 6. A lightning strikeinto a receptor 6 will hence result in a lightning current beingcompletely or partially conducted from the receptor through the resistorelement 10 and on to ground 9. When an electric current is conductedthrough a resistor, energy in the form of heat deposits, ie thelightning current heats the resistor element 10. Besides, the system 8comprises not shown means for measuring an increase in temperature inthe resistor element 10—albeit an example of this is shown in FIG. 4,said means for measuring being coupled to means for registering anincrease in temperature, eg data processing equipment 12. The dataprocessing equipment 12 may also comprise means for storing data, meansfor registering a point in time, means for dispatching an electronicmessage—directly or wirelessly, means for alerting, eg in the event of aspecific increase in temperature, etc. Preferably, the system 8 can beenclosed in a thermally insulating material 11, whereby the heating ofthe resistor element 10 is not unintentionally affected by externalfactors, whereby an accurate and reliable measurement is accomplished.In case of a lightning strike in one of the five receptors 6 of theblade, it will thus be possible to read an increase in temperature ofthe resistor element 10 that can be processed via the mentioned dataprocessing equipment 12, including be converted to an expression of atleast the magnitude of the amount of energy in a lightning currentcaused by lightning strike.

FIG. 3 shows a variety in which each receptor 6 is in direct connectionwith a system for determining the magnitude of the amount of energycontained in a lightning strike. Each receptor is connected to aresistor element 10 and further connected to a connection to ground 9.In the Figure it is shown that each resistor element 10 is connected—vianot shown means for measuring an increase in temperature—to each theirdata equipment 12 (in a manner that corresponds essentially to thatmentioned above in the context of the system 8 shown in FIG. 2), but ofcourse the system can also be constructed such that centrally arrangeddata equipment 12 can handle input from a number of different resistorelements 10. Moreover, it is not required that data processing equipment12 is to be present in each blade 5 of a wind energy plant; theequipment can be arranged in one of the three blades 5 or in the toweror in the nacelle or in any other location in proximity of the plant andfrom there receive input from all measurements of lightning strikes ineach receptor 6 on any of the blades 5.

As shown in FIG. 3, resistor elements 10 can be mounted in directconnection with each receptor 6 in the blade 5, whereby it is possibleto monitor the individual receptor 6 and to register lightning strikesand their amount of energy grounded from the receptor 6 in question.

It is moreover possible to arrange all resistor elements 10 assembled ineg the blade root along with the necessary data processing equipment 12.By this solution a lightning grounding cable 7 is taken from eachreceptor 6 to each their resistor element 10 and from there it isconducted to ground 9.

FIG. 4 shows an embodiment of a resistance element 10 according to theinvention. The resistor element 10 is configured as a rotationallysymmetrical object having an increased diameter at the ends 13, whichends 13 are configured in this manner in consideration of the selectedtype of terminal. The shown type of terminal is configured as sleeves 14with an external thread 15 mounted at the ends of each lightninggrounding cable 7 for cooperation with and connection to the resistorelement 10 via threaded bores configured at the ends 13 of the resistorelement. In this manner it is ensured that there is sufficient contactface for a lightning current to be transferred from the cable 7 to theresistor element 10 and from then on without a welding occurring betweenthe sleeves 14 and the resistor element 10. It will be obvious to theperson skilled in the art that other types of terminals can beused—according to preference—for such joints.

Moreover, FIG. 4 shows a way in which a thermo-element can be attachedto the resistor element 10. The shown solution comprises a hole boredinto the resistor element 10 for receiving the thermo-element 16. Thethermo-element 16 is fixated with a screw 17, whereby a clamp joint isprovided. This solution, too, can be provided in a variety of ways. Thefundamental is that good contact is ensured between the resistor element10 and the thermo-element 16 for transfer of heat. This contact isfurther ensured by use of a heat-conductive paste in the joint. Thethermo-element 16 can be connected to a not shown electronicthermometer, which thermometer can be connected to data processingequipment 12, eg a computer unit. Alternatively the thermo-element 16can be connected directly to data processing equipment 12. For instancethe thermometer or the computer unit can be configured for monitoringand registering that an increase in temperature occurs, eg beyond apre-determined threshold value.

The term ‘monitoring’ is intended to designate that it is constantlychecked whether an increase in temperature occurs. By ‘registration’ ismeant that an increase in temperature is found or detected. By‘measurement of increase in temperature’ is meant that the size and/orchange of a parameter is found and determined, respectively.

Herein the resistor element 10 is shown with an external thermalinsulation 11 outlined schematically by the dotted markings. Thisinsulation 11 ensures a more precise registration of a flash, thedifference in temperature being measurable with a much higher degree ofprecision, when the surroundings emit less heat, and likewise theinfluence from the surroundings is, at a minimum, reduced.

According to a convenient embodiment of the invention of the blade of awind turbine 5 of a wind energy plant 1, a resistor element 10 isemployed which is rotationally symmetrical and has a length of 200 mmand a cylindrical central section with a diameter of 11.5 mm and has twocylindrical ends with a diameter of 20 mm, said ends each having alength of 50 mm. Moreover, each end is provided with an internal,concentric threaded bore having a depth of 45 mm provided with M10thread. The M10 thread suits standard cable boxes for coupling withlightning grounding cables 7 from a receptor 6 and a connection toground 9, respectively, and the depth of the thread enables the cablebox to be introduced deeply enough for a sufficient contact area to beaccomplished for transfer of the lightning current. The increaseddiameter of the ends compared to the central section serves the purposeof preventing the internal threaded bores from reducing the crosssectional area too much—likewise with a view to conducting the lightningcurrent. Centrally on the middle of the central section a through-boreis provide having a diameter of 3 mm for receiving a thermo-element 16.Perpendicular to the transverse bore a threaded bore with M3 thread isprovided, wherein a fixation screw is provided for ready fixation of athermo-element 16. The resistor element 10 can be manufactured fromNirosta® (X5CrNi 18-10) available from Thyssen Krupp. A suitablyselected thermo-element 16 is arranged and secured in the opening in thecentral section of the resistor element 10 and is connected to an analogmeasurement converter type 231, available from LKM Electronic GmbH inGermany and having the property that the thermo-element 16 isgalvanically separated from the electronics part of the measurementconverter, whereby lightning current is not conducted from the resistorelement 10 into the electronics part. The output of the measurementtransformer is a norm signal of 4-20 mA. The output of the measurementtransformer is coupled to a computer for further processing ofmeasurements. The measurement transformer and the computer are arrangedwithin a metal cabinet that serves as a Faraday cage and protects themeasurement transformer and the computer. A power supply cable for themeasurement transformer and the computer in the form of a shielded cableor a cable drawn in a metal tubing is advanced to the blade 5 from thehub 4 and further along with the lightning grounding cable 7. Theabove-mentioned design of the resistor element 10 will enablemeasurement of increases in temperature for typical lightning currentsof up to about 400 K.

-   -   The unit “mm” is millimetre.    -   The unit “mA” is milli Ampere    -   The unit “K” is Kelvin.        The embodiments shown in FIGS. 1-3 are implementations in        connection with up-to-date wind energy plants, but of course the        person skilled in the art could easily implement aspects of the        invention in other contexts where it is desired to register        lightning strikes and determine the fierceness of lightning        strikes, respectively, such as in large machine or factory        plants, buildings, towers, bridges, etc.

It will thus be understood that the invention as disclosed in thepresent description and figures can be modified or changed, whilecontinuing to be comprised within the scope of protection of thefollowing claims.

1. A method of registering at least one lightning strike in the blade(5) of a wind turbine, characterised in that the method comprises thatthe lightning strike is captured by a receptor (6) in the blade of thewind turbine, from where a lightning current is completely or partiallyconducted through at least one electric resistor (10), thereby heatingit; and that the lightning strike is registered on the basis of theincrease in temperature of the resistor; and that the at least onecharacteristic of at least one lightning strike is determined on thebasis of the increase in temperature of the resistor, saidcharacteristic comprising the amount of energy contained in thelightning strike determined on the basis of a measurement of themagnitude of the increase in temperature.
 2. A method according to claim1, characterised in that the point in time of the lightning strike isdetermined on the basis of a measurement of the point in time of theincrease in temperature.
 3. A method according to claim 1, characterisedin that the determination of the amount of energy contained in thelightning strike on the basis of the magnitude of the increase intemperature of the resistor is performed by use of a pre-calculated ormeasured ratio coefficient that defines the ratio between an increase intemperature in the electric resistor (10) and the amount of energy ofthe lightning current conducted through the electric resistor (10).
 4. Amethod according to claims 1, characterised in that said characteristicis used as an indicator for assessing the potential extent of damagemade by said lightning strike in the blade (5) of the wind turbine.
 5. Awind turbine (1) comprising means for grounding a lightning current,including at least one receptor (6) and at least one groundingconnection (7) from the receptor to an external connection to ground(9), characterised in that the wind turbine comprises means (16) formeasuring an increase in temperature in at least one electric resistor(10) and means for determining the amount of energy contained in alightning strike on the basis of the magnitude of the increase intemperature, wherein the resistor (10) is connected to the receptor (6)or to the grounding connection (7) in a position between the receptor(6) and the connection to ground (9), preferably by being insertedserially in the grounding connection (7) and serially inserted betweenthe grounding connection (7) and the receptor (6), respectively, or bybeing incorporated into a measurement shunt, a measuring bridge or otherparallel circuit connected to the grounding connection (7) or to thereceptor (6).
 6. A wind turbine (1) according to claim 5, characterisedin that the resistor (10) and the receptor (6) are interconnected ormade integrally.
 7. A wind turbine (1) according to claim 5,characterised in that the at least one electric resistor (10) ispreferably arranged in a blade (5) of the wind turbine (1).
 8. A windturbine (1) according to claim 5, characterised in that the wind turbinecomprises a number of receptors (6), said receptors being each connectedto an external connection to ground (9); and that at least one electricresistor (10) is connected to each receptor (6) or to a groundingconnection (7) between each receptor (6) and the connection to ground(9) to which the receptor is connected; and that the wind turbinecomprises means for measuring (16) an increase in temperature in eachelectric resistor.
 9. A wind turbine (1) according to claim 8characterised in that the wind turbine comprises means for storing atleast one of the parameters comprising the measured increase intemperature, a determined amount of energy and a determined point intime.
 10. A wind turbine (1) according to claim 8, characterised in thatmeans for measuring (16) the increase in temperature in the electricresistor (10) comprises an electronic thermometer comprising athermo-element (16), which thermo-element is arranged in thermallyconductive contact with the electric resistor.
 11. A wind turbine (1)according to claim 8, characterised in that means for measuring theincrease in temperature in the electric resistor (10) comprises aninfrared thermo-sensor and a camera for infrared recording,respectively, means for measuring a temperature-related change inresistance in the electric resistor, a non-touch temperature sensor, anoptical fibre or some other kind of equipment for measuring an increasein temperature.
 12. A wind turbine (1) according to claim 5,characterised in that the wind turbine comprises means for determining apoint in time of a lightning strike on the basis of a point in time ofan increase in temperature.
 13. A wind turbine (1) according to claim 5,characterised in that at least the electric resistor (10) is essentiallyenshrouded in thermally insulating material (11).
 14. A wind turbine (1)according to claim 5, characterised in that the wind turbine comprisesmeans (12) for monitoring and storing registrations of lightningstrikes, including optionally also characteristics of lightning strikes,said means comprising a computer unit arranged in direct or wirelessconnection with means (16) for measuring the increase in temperature,said computer unit being preferably arranged in or at the blade (5) ofthe wind turbine, including in the blade (5) of a wind turbine.
 15. Awind turbine (1) according to claim 5, characterised in that the windturbine comprises means of alerting or halting the wind turbine at agiven increase in temperature in the resistor (10).
 16. A wind turbine(1) according to claim 5, characterised in that the wind turbinecomprises means for dispatching an electronic message, said messagecomprising data relating to the increase in temperature.
 17. A windturbine (1) according to claim 5, characterised in that the wind turbinecomprises means for registering a lightning current, including alightning registration card comprising at least one magnet strip.
 18. Awind turbine (1) according to claim 5, characterized in that theresistor is configured essentially an elongate object having at its ends(13) an increased expanse transversally to its longitudinal axis.
 19. Awind turbine (1) according to claim 5, characterized in that theresistor is essentially rotationally symmetrical about its longitudinalaxis and comprises an opening adapted for receiving a thermo-element(16).
 20. A wind turbine (1) according to claim 5, characterized in thatthe resistor is manufactured from steel, preferably stainless steel. 21.A system (8) for use in the registration of at least one lightningstrike in a blade of a wind turbine, said system comprising means forgrounding a lighting current, including at least one receptor (6) formounting in the blade (5) of the wind turbine, and at least onelightning grounding cable (7) coupled to the receptor (6), characterisedin that the system comprises means (16) for measuring an increase intemperature in at least one electric resistor (10), and characterized inthat the system comprises means for determining the amount of energycontained in a lightning strike on the basis of the magnitude of theincrease in temperature, wherein the at least one resistor is adapted tobe coupled to the lightning grounding cable (7) or to the receptor (6)and be inserted between the ground (9) and the receptor (6),respectively, in such a manner that the resistor will be heated by alightning current.
 22. A system (8) according to claim 21, characterisedin that the resistor (10) is inserted serially in the lightninggrounding cable (7) and inserted serially between the ground (9) and thereceptor, respectively, or is incorporated into a measurement shunt,measuring bridge or other parallel circuit connected to the groundingconnection or a receptor (6).